The conventional water heater consists of a tank to contain water to be heated and a jacket is spaced outwardly of the tank and the upper end of the jacket is enclosed by a sheet metal cover or lid. In the past, fiber glass insulation was applied in the space between the tank and jacket and more recently, foamable or expandable resins, such as polyurethane resin, have been employed as the insulation.
In the normal foaming operation, the liquid resin is introduced into the space between the tank and the jacket through an opening in the cover. As the resin expands, a substantial pressure is exerted upwardly against the cover. To resist this pressure of the expanding resin, the peripheral flange of the cover has been fastened to the upper edge of the jacket by screws, but even with this attachment the pressure of the expanding resin may tend to deform the cover, providing an unattractive appearance for the water heater.